With the increasing requirements on crude oil, crude oil is continuously explored; light crude oil resource gradually decreases and the ratio of heavy and inferior acid-containing crude oil is increasing. The quality of the world crude oil generally tends to become heavy and inferior, and the output of crude oil having high sulfur content, high total acid number, high metal content and high carbon residue is rapidly increased. It is predicted that the world output of inferior acid-containing crude oil will be increased from currently 16% to 20% in 2010. Based on this background, to process inferior acid-containing crude oil with the catalytically cracking apparatus is the problem which we have to confront.
Inferior acid-containing crude oil has low hydrogen content, high content of metals, such as Ni, V and the like, high content of condensed aromatics, high content of S and N, high density and high conradson carbon number, so that it is difficult to crack said crude oil. Since it is difficult to crack the inferior acid-containing crude oil, the catalytic cracking equipment for processing such crude oil is forced to throw off a great quantity of slurry, with the result that the total yield of liquid products (liquefied gas, gasoline, diesel oil) decreases. Higher Ni and V content renders that hydrogen content in the products are greatly increased, which has strong destruction on the catalyst. In order to maintain the balance activity of the catalyst, the unit consumption of the catalyst will be certainly increased. Moreover, the addition of the metal deactivator is also difficult to achieve the desirable effect. Thus it is advisable that such inferior acid-containing crude oil should be upgraded or pretreated before processing, so as to increase the hydrogen/carbon ratio of the crude oil and reduce the metal content and conradson carbon number.
In order to improve the properties of crude oil and to reduce the conradson carbon number and metal content, a non-catalytic pretreating process comprises introducing crude oil having a substantial conradson carbon number and metals content into the demetallization and decarbonizing zone of the fluidized catalytic cracking apparatus to come into contact with an inert fluidizable solid particles. Under the conditions of at least 480° C. and less than 2 s of the contact time, high boiling components of the crude oil and metals are deposited on the inert fluidizable solid particles. These particles are recycled into a burning zone to remove combustible deposit. Inert particles from which combustible deposit is burned away are recycled into demetallization and decarbonizing zone to come into contact with crude oil again. Said crude oil treated with such process can be used as the feedstock of FCC.
Recently, the process of inferior acid-containing crude oil is gradually drawing increasing attention. The amount of highly acidic crude oil in the international market is gradually increased. In 2005, the world output of highly acidic crude oil occupied 5.5% of the total output of crude oil. Acidic substances in crude oil are inorganic acids, phenols, mercaptans, aliphatic carboxylic acids, naphthenic acids and the like, wherein naphthenic acids are the uppermost acidic oxides in crude oxide and occupy about 90 wt % of acidic oxides. Studies show that crude oil having a total acid number of 0.5 mgKOH/g will result in significant corrosion of the refining equipments. Thus crude oil having a total acid number of greater than 0.5 mgKOH/g is called as highly acidic crude oil. During the processing of crude oil, naphthenic acids in crude oil may directly react with iron, thereby causing corrosion of furnace tube, heat exchanger and other refining equipments. In addition, naphthenic acids may react with the protective film FeS on the petrolatum equipments to enable the metal equipment to expose new surface and to be subjected to new corrosion. For general petroleum products, such as gasoline, diesel oil and kerosene, there are always the requirements on the acid number in the quality indexes thereof. Overhigh acid number will bring the same corrosion problem to the final customers.
The processes introduced in the documents and patents further include the physical adsorption process, the thermal treatment, the thermal cracking process and the catalytic hydrogenation process. These processes, however, are not applied in the practical application.
Physical adsorption process—In the presence of an adsorbent, acid-containing crude oil or fraction is thermally treated at 250-350° C. to adsorb and transfer acid-containing compounds in crude oil, wherein the adsorbent may be a waste catalytically cracking catalyst. Alternatively, the blend of crude oil and alkaline earth metal oxides is heated at 100-300° C. to enable said alkaline earth metal oxides to react with organic acids or sulfides in crude oil, so as to produce the precipitates of alkaline earth metal carbonates and alkaline earth metal sulfides. After separation, crude oil from which naphthenic acids and sulfides are removed is obtained.
Catalytic hydrogenation process—Under the conditions of a hydrogen partial pressure of 2-3 Mpa and a reaction temperature of 250° C., acid-containing crude oil is hydrotreated by using Ni—Mo or Ni—Co hydrorefining catalyst in which the carrier is alumina, so as to decompose naphthenic acids to CO, CO2, H2O and petroleum hydrocarbons having a low molecular weight, and to reduce the total acid number of crude oil from 2.6 mgKOH/g to 0.15 mgKOH/g. Although catalytic hydrogenation process has a better deacidification effect, it needs a high pressure-resistant and high temperature-resistant apparatus, and hydrogen gas. Said apparatus thus requires a high apparatus investment and a large technological investment. In the absence of hydrogen gas, highly acidic crude oil can be treated with Ni—Mo or Ni—CO hydrorefining catalyst at 285-345° C., so as to reduce the total acid number of crude oil from 4.0 mgKOH/g to 1.8 mgKOH/g.
Thermal treatment and thermal cracking process (including catalytically thermal cracking)—U.S. Pat. No. 5,891,325 discloses a process for reducing total acid number of crude oil by a multistage thermal reaction. The thermal reaction in said process comprises a plurality of stages, wherein each stage of the thermal reaction decomposes at a certain temperature and pressure a portion of petroleum acids, and produces volatile organic acids, petroleum hydrocarbons and non-volatile petroleum hydrocarbons. While the reaction is carried out, said process further comprises sweeping the reaction system with an inert gas, collecting volatile components, neutralizing most of organic acids with a basic salt of a Group HA metal, such as CaO, Ca(OH)2, CaCO3, MgO and the like, to produce volatile petroleum hydrocarbons. Then said volatile petroleum hydrocarbons and non-volatile petroleum hydrocarbons are blended together to obtain crude oil from which the petroleum acids are removed.
CN1827744A discloses a method for processing crude oil with high total acid number, comprising preheating said crude oil which has a total acid number of more than 0.5 mgKOH/g after the pretreatment and injecting them into the fluidized catalytic cracker to contact the catalyst, and reacting in catalytically cracked reaction condition, naphthenic acid in said crude oil being cracked to be hydrocarbons and CO2, separating the reacted oil gas and the catalyst, in which the oil gas is fed into the subsequent separation system, while the reacted catalyst can be circularly used after stripping and regenerating.
At present, inferior acid-containing crude oil is generally processed by mixing with low acid crude oil. Generally, the total acid number of the mixed crude oil is required to be not more than 0.5 mgKOH/g, so that the blending ratio of acid-containing crude oil is limited to some extent. The higher the total acid number, density and carbon residue of acid-containing crude oil are, the more difficult the processing thereof is.